The small metal grille on the stable wall separating Kauto Star and Denman looks worn and battered by brotherly affection. These two champion horses have won the last three Gold Cups between them, and they contest the same race next Friday, but in their adjoining stables in Somerset the racing rivals show a touching bond.

Denman, the brooding opposite to a preening Kauto Star, is infamous for turning his hulking rear on unwanted visitors. But on a quiet and sunlit afternoon he reveals a more personal habit by ambling over so that he can knock on the grille to summon his neighbour.

Lucinda Gould, the sparky 24-year-old stable lass who looks after Denman, stands inside her horse's stable. "They're almost like brothers," she says. "Denman will walk up to the grille when he wants a chat with Kauto. He'll give it a tap and Kauto will come and say hello. And, just as often, it'll be Kauto knocking for Denman. We all know how different they are as horses. But the strange thing is they could hardly be closer here."

Gould pats the huge beast on his muscled rump. "If Denman was a person," she says, grinning, "he'd be a Guinness-drinking Irish rugby player whose bottom sticks out of his jeans. He's a bit of a lad. But Kauto is a real French poseur. He just has to see a camera to pull a pose."

Slipping into her best impersonation of a celebrity horse imitating a supermodel, Gould pouts artfully while resting a hand on her jutting hip. "That's Kauto. He loves the attention. Denman can be surly but people make the mistake of thinking he's thick. He's not. Look at the way he deals with the press."

Gould's laugh jangles across the yard. "Listen to me. I'll be telling you he's calling his agent next. But on our media day the other week he was such a pro. He came out into the yard and let the photographers do their stuff. But then he'd had enough. All they could see of him the rest of the day was his very big bottom."

In the box next door, Nick Child, a 22-year-old stable lad from Islington, cares for arguably the greatest horse in jump-racing today, and the clear favourite to win the race for a third time on Friday. His friendship with Gould is built on a bantering rivalry but he echoes her argument: "They're both so intelligent. That fella, Denman, is huge and, like George Foreman when he was world champion, he can look menacing. But my fella, Kauto, is definitely Muhammad Ali. He has such glamour and charisma."

Kauto Star chooses that exact moment to untether himself from a big hook on the wall. Expertly using his teeth to untie the rope he slips free and wanders towards us. "Look at that," Child exclaims. "That's the first time he's untied himself. It's almost as if he could tell we were saying he's so smart. He probably also wants to see if there's a camera around because he really does know how good he is. He's so athletic and a fantastic jumper And Kauto is always eager to put on a show.

"Lucinda's also right about Denman. If he was human he'd drink lots of beer and wolf down packets of crisps whereas my lad would choose fruit and water. And so that's why I pay him such respect as a champion. Clifford Baker [the head lad who runs the yard for trainer Paul Nicholls] always reminds us that the more races Kauto wins now the more history he makes. I think he's on course for something special on Gold Cup Day – which just happens to be his birthday."

It will be Kauto Star's 10th "birthday" while Child has his 23rd birthday two days earlier. That close-knit intimacy is sealed by Denman turning 10 next month. These two icons of National Hunt racing will not be able to entrance us much longer. Denman, in particular, has some severe questions to confront at Cheltenham. Even his astonishing victory in the 2008 Gold Cup serves as a reminder of all he has endured the last few years. The toll of that day is also a warning not to become too sentimental about a brutal business.

Dan Skelton, who assists Nicholls and lives in a small house on the yard, paints a graphic picture of Denman's post-race exhaustion in 2008. "The last thing at night I'll go round and check on the horses," Skelton says. "It can be quite awe-inspiring to be alone with them but normally you never catch Denman lying down. But after the 2008 Gold Cup he lay down every night for three weeks. It was a sure sign of how much that race took out of him. He walked away from the winners' enclosure at Cheltenham, having beaten Kauto, and looked quite bright. But back in the paddock he was so hot. It took a couple of hours hosing him down to cool him off. He pours everything into winning."

It seems obvious that there might be a link between that epic performance and the heart problems Denman encountered a few months later. "When he came back from the [heart] treatment you could see it had hit him for six," Skelton says. "People didn't realise how close we got to pulling up stumps on him. But he's so tough and gritty that he came back and last year, in the 2009 Gold Cup, he ran the race of all races to finish second to Kauto. Most horses will never finish second in the Gold Cup after the best run of their career. But Denman did it on the back of one of his worst preparations."

Baker knows both horses better than anyone, and he has been responsible for 45 Festival winners. He rides Kauto Star every day and is on the back of Denman at least once a week. "Denman is more aggressive but lazier in training. He wouldn't go up our gallops if you let him get away with it. Denman would just stand at the bottom whereas Kauto loves going up there. But on race day Denman gives everything and that's why we have to make sure he is super-fit."

Over the last few weeks Nicholls and Baker have worked Denman much more rigorously than his stable-mate – for Kauto Star looks a picture of racing vitality. The defending champion enters the Gold Cup on the back of a majestic performance in his last race, the King George at Kempton on Boxing Day. Denman, meanwhile, had a disastrous ending to the Aon Chase at Newbury a month ago today when he made two bad mistakes and unseated his rider, Tony McCoy.

Baker dismisses fears that Denman has ruined his Gold Cup chance. "Horses change a lot in a month. Kauto proved that after the Betfair last November [when he just beat Imperial Commander]. He was simply imperious the following month at Kempton. I thought his Gold Cup performance last year was as good as anything we'd ever see – but you'd have to say he was marginally better at Kempton. He shouldn't be winning the King George by 10 or 20 lengths – let alone 36 lengths.

"But Kauto is probably the best horse any of us in this generation will ever come across. It's not like he's finished his career and we're saying, 'Oh, he's won two Gold Cups and four King Georges'. He's odds-on favourite for the Gold Cup next week and he looks to be in his prime.

"But we also love Denman and I can promise you he's going to be ready. Right now you could say Kauto has the clear edge in form and technique. But Denman will be absolutely up for it. The battle is definitely on and of their three Gold Cup races this could be the best of the lot."

Baker is not given to exaggeration and his anticipation of a defining race is shared by Child and Gould – who will travel together with their horses, in the same box, on Gold Cup morning. "We're very lucky," Child says. "Some people spend their whole lives in racing and they don't come near horses as good as this. We also get to see the horses in close-up on race day. Denman will be quiet but Kauto will be a bit more worked up. He just wants to get out there and perform."

Gould lets out a deep breath. "I get very nervous. You feel such affection for them. But then, once you get to Cheltenham and you lead them out, it's like no other feeling. When I'm with Denman, and Nick's with Kauto, it's like we're walking out with two celebrities. People are fascinated by them. But we know that they're more than superstars. They're best mates.

"When Denman went away for a week for his heart treatment Kauto really missed him. He was walking around his box, looking lost, and every now and then he'd tap on the grille but get no answer. So we eventually moved Noland into Denman's box. But Kauto just looked at Noland as if to say, 'Who's this bugger?'

"And when Denman came back and went racing again it was really sweet. Denman won the Hennessy [last November] with a great run and when I brought him back to the yard Kauto was waiting for us. His ears were pricked and he stared at Denman as if he was saying, 'Well done, mate. I saw you on the telly and you were brilliant.'"

Child makes another vivid point: "Close to race day Kauto will some times take a bite at Denman. There's no nastiness in him, and he could never be a mean horse, but that also tells you that Kauto is ready. He wants to beat his mate. He wants that third Gold Cup."

As the two young stable hands wander off to look after some of the other horses in Nicholls's yard, Denman and Kauto Star amble over. Each of them looks out with real interest over the bottom half of their door, as if they might be counting down the days before they race each other up the daunting hill at Cheltenham. Kauto Star appears enthused, while Denman offers a more serious presence.

In that hushed moment, it's easy to imagine one of them walking up to the grille on the wall early next Saturday to give it a little tap. And, on the morning after Gold Cup day, a tap on the grille will spell out either congratulation or consolation – from one champion to another.